[InstCombine] Allow splats with poison/undef in llvm::decomposeBitTestICmp

This change is made to enable conversion of a masked icmp splat vector
containing poison/undef to an equality expression.

llvm::decomposeBitTestICmp Alive2 correctness examples using splat/masking vectors:

    SLT <    https://alive2.llvm.org/ce/z/pPTTHh
    SLE <=   https://alive2.llvm.org/ce/z/qQhAmU
    SGT >    https://alive2.llvm.org/ce/z/koFHzF
    SGE >=   https://alive2.llvm.org/ce/z/3SNz2S
    ULT <u   https://alive2.llvm.org/ce/z/W8ktzQ
    ULE <=u  https://alive2.llvm.org/ce/z/G5SdUY
    UGT >u   https://alive2.llvm.org/ce/z/WFwYxq
    UGE >=u  https://alive2.llvm.org/ce/z/DzJszP

Tests have been verified using Alive2:

    icmp-logical.ll: @nomask_splat_and_B_allones       https://alive2.llvm.org/ce/z/zmJwQU
    icmp-logical.ll: @nomask_splat_and_B_mixed         https://alive2.llvm.org/ce/z/ktzgzd
    signed-truncation-check.ll: @positive_vec_undef0   https://alive2.llvm.org/ce/z/-sTRLD

Differential Revision: https://reviews.llvm.org/D143032